The ductus arteriosus normally closes within 24 hours after delivery in full-term newborn infants. Following the functional constriction of the vessel, there is an anatomic occlusion due to migration of smooth muscle cells from the inner muscle media into a neointima. In preterm infants, this anatomic remodeling does not occur, and the infants frequently develop a patent ductus arteriosus. This is associated with significant morbidity bronchopulmonary dysplasia, necrotizing enterocolitis, etc. We used immunohistochemistry to study the pattern of integrin receptors on smooth muscle cells (SMCS) of the ductus obtained from fetal, 2-day, and 4-day-old newborn rhesus monkeys. Smcs use integrin receptors to attach to and migrate over the extracellular matrix as they remodel the vessel. SMCS of the fetal ductus possess most of the integrin receptors in vivo that we previously observed in culture. In contrast, the a5b1 and avb3 integrins, which are found in culture, were not observed in vivo. After delivery, several unique changes in the integrins are expressed by the migrating smcs that form the neointima. These migrating smcs express the a5b1 and avb3 integrins, just like smcs in culture. We have previously observed that the a5b1 and avb3 integrins play an essential role in SMC migration in vitro. Therefore, as stationary smcs in vivo are induced to migrate into the neointima, they begin to express the same integrins that are functionally necessary for smc migration in vitro. We hypothesize that in the preterm ductus, where there is no anatomic remodeling, there will be a lack of expression of these promigratory integrins. Determining what factors induce integrin expression will help to uncover why the preterm ductus fails to remodel after birth.